Postfix
Virtual Domain Hosting Howto

Purpose of this document

This document requires Postfix version 2.0 or later.

This document gives an overview of how Postfix can be used for
hosting multiple Internet domains, both for final delivery on the
machine itself and for the purpose of forwarding to destinations
elsewhere.

The text not only describes delivery mechanisms that are built
into Postfix, but also gives pointers for using non-Postfix mail
delivery software.

Most Postfix systems are final destination for only a
few domain names. These include the hostnames and [the IP addresses]
of the machine that Postfix runs on, and sometimes also include
the parent domain of the hostname. The remainder of this document
will refer to these domains as the canonical domains. They are
usually implemented with the Postfix local domain address class,
as defined in the ADDRESS_CLASS_README file.

Besides the canonical domains, Postfix can be configured to be
final destination for any number of additional domains.
These domains are called hosted, because they are not directly
associated with the name of the machine itself. Hosted domains are
usually implemented with the virtual alias domain address class
and/or with the virtual mailbox domain address class, as defined
in the ADDRESS_CLASS_README file.

But wait! There is more. Postfix can be configured as a backup
MX host for other domains. In this case Postfix is not the final
destination for those domains. It merely queues the mail when
the primary MX host is down, and forwards the mail when the primary
MX host becomes available. This function is implemented with the
relay domain address class, as defined in the ADDRESS_CLASS_README
file.

Finally, Postfix can be configured as a transit host for sending
mail across the internet. Obviously, Postfix is not final destination
for such mail. This function is available only for authorized
clients and/or users, and is implemented by the default domain
address class, as defined in the ADDRESS_CLASS_README file.

The examples in this text use table lookups from local files
such as DBM or berkeley DB. These are easy to debug with the
postmap command:

Example: postmap -q info@example.com hash:/etc/postfix/virtual

See the documentation in LDAP_README, MYSQL_README and PGSQL_README
for how to replace local files by databases. The reader is strongly
advised to make the system work with local files before migrating
to network databases, and to use the postmap command to verify
that network database lookups produce the exact same results as
local file lookup.

The simplest method to host an additional domain is to add the
domain name to the domains listed in the Postfix mydestination
configuration parameter, and to add the user names to the UNIX
password file.

This approach makes no distinction between canonical and hosted
domains. Each username can receive mail in every domain.

In the examples we will use "example.com" as the domain that is
being hosted on the local Postfix machine.

Line 2: the virtual_alias_domains setting tells Postfix
that example.com is a so-called virtual alias domain. If you omit
this setting then Postfix will reject mail (relay access denied)
or will not be able to deliver it (mail for example.com loops back
to myself).

Lines 3-8: the /etc/postfix/virtual file contains the virtual
aliases. With the example above, mail for postmaster@example.com
goes to the local postmaster, while mail for info@example.com goes
to the UNIX account joe, and mail for sales@example.com goes to
the UNIX account jane. Mail for all other addresses in example.com
is rejected with the error message "User unknown".

Line 10: the commented out entry (text after #) shows how
one would implement a catch-all virtual alias that receives mail
for every example.com address not listed in the virtual alias file.
This is not without risk. Spammers nowadays try to send mail from
(or mail to) every possible name that they can think of. A catch-all
mailbox is likely to receive many spam messages, and many bounces
for spam messages that were sent in the name of anything@example.com.

Execute the command "postmap /etc/postfix/virtual" after
changing the virtual file, and execute the command "postfix
reload" after changing the main.cf file.

Note: virtual aliases can resolve to a local address or to a
remote address, or both. They don't have to resolve to UNIX system
accounts on your machine.

More details about the virtual alias file are given in the
virtual(5) manual page, including multiple addresses on the right-hand
side.

Virtual aliasing solves one problem: it allows each domain to
have its own info mail address. But there still is one drawback:
each virtual address is aliased to a UNIX system account. As you
add more virtual addresses you also add more UNIX system accounts.
The next section eliminates this problem.

As a system hosts more and more domains and users, it becomes less
desirable to give every user their own UNIX system account.

With the Postfix virtual(8) mailbox delivery agent, every
recipient address can have its own virtual mailbox. Unlike virtual
alias domains, virtual mailbox domains do not need the clumsy
translation from each recipient addresses into a different address,
and owners of a virtual mailbox address do not need to have a UNIX
system account.

The Postfix virtual(8) mailbox delivery agent looks up the user
mailbox pathname, uid and gid via separate tables that are searched
with the recipient's mail address. Maildir style delivery is turned
on by terminating the mailbox pathname with "/".

If you find the idea of multiple tables bothersome, remember
that you can migrate the information (once it works), to an SQL
database. If you take that route, be sure to review the "local files versus databases"
section at the top of this document.

Line 2: The virtual_mailbox_domains setting tells Postfix
that example.com is a so-called virtual mailbox domain. If you omit
this setting then Postfix will reject mail (relay access denied)
or will not be able to deliver it (mail for example.com loops back
to myself).

Line 3: The virtual_mailbox_base parameter specifies a
prefix for all virtual mailbox pathnames. This is a safety mechanism
in case someone makes a mistake. It prevents mail from being
delivered all over the file system.

Lines 4, 10-15: The virtual_mailbox_maps parameter specifies
the lookup table with mailbox (or maildir) pathnames, indexed by
the virtual mail address. In this example, mail for info@example.com
goes to the mailbox at /var/mail/vhosts/example.com/info while mail
for sales@example.com goes to the maildir located at
/var/mail/vhosts/example.com/sales/.

Line 5: The virtual_minimum_uid specifies a lower bound
on the mailbox or maildir owner's UID. This is a safety mechanism
in case someone makes a mistake. It prevents mail from being written
to sensitive files.

Lines 6, 7: The virtual_uid_maps and virtual_gid_maps
parameters specify that all the virtual mailboxes are owned by a
fixed uid and gid 5000. If this is not what you want, specify
lookup tables that are searched by the recipient's mail address.

Line 14: The commented out entry (text after #) shows how
one would implement a catch-all virtual mailbox address. Be prepared
to receive a lot of spam, as well as bounced spam that was sent in
the name of anything@example.com.

NEVER put a virtual MAILBOX wild-card in the virtual ALIAS
file!!

Lines 8, 17, 18: As you see, it is possible to mix virtual
aliases with virtual mailboxes. We use this feature to redirect
mail for example.com's postmaster address to the local postmaster.
You can use the same mechanism to redirect an address to a remote
address.

Line 18: This example assumes that in main.cf, $myorigin
is listed under the mydestination parameter setting. If that is
not the case, specify an explicit domain name on the right-hand
side of the virtual alias table entries or else mail will go to
the wrong domain.

Execute the command "postmap /etc/postfix/virtual" after
changing the virtual file, execute "postmap /etc/postfix/vmailbox"
after changing the vmailbox file, and execute the command "postfix
reload" after changing the main.cf file.

Note: mail delivery happens with the recipient's UID/GID
privileges specified with virtual_uid_maps and virtual_gid_maps.
Postfix 2.0 and earlier will not create mailDIRs in world-writable
parent directories; you must create them in advance before you can
use them. Postfix may be able to create mailBOX files by itself,
depending on parent directory write permissions, but it is safer
to create mailBOX files ahead of time.

More details about the virtual mailbox delivery agent are given
in the virtual(8) manual page.

While non-Postfix software is being used for final delivery,
some Postfix concepts are still needed in order to glue everything
together. For additional background on this glue you may want to
take a look at the virtual mailbox domain class as defined in the
ADDRESS_CLASS_README file.

The text in this section describes what things should look like
from Postfix's point of view. See LMTP_README or MAILDROP_README
for specific information about Cyrus or about Courier maildrop.

Here is an example for a hosted domain example.com that delivers
to a non-Postfix delivery agent:

Line 2: With delivery to a non-Postfix mailbox store for
hosted domains, the virtual_transport parameter usually specifies
the Postfix LMTP client, or the name of a master.cf entry that
executes non-Postfix software via the pipe delivery agent. Typical
examples (use only one):

Postfix comes ready with support for LMTP. And an example
maildrop delivery method is already defined in the default Postfix
master.cf file. See the MAILDROP_README document for more details.

Line 3: The virtual_mailbox_domains setting tells Postfix
that example.com is delivered via the virtual_transport that was
discussed in the previous paragraph. If you omit this
virtual_mailbox_domains setting then Postfix will either reject
mail (relay access denied) or will not be able to deliver it (mail
for example.com loops back to myself).

Lines 4, 7-13: The virtual_mailbox_maps parameter specifies
the lookup table with all valid recipient addresses. The lookup
result is ignored by Postfix. In the above example, info@example.com
and sales@example.com are listed as valid addresses, and mail for
anything else is rejected with "User unknown". If you intend to
use LDAP, MySQL or PgSQL instead of local files, be sure to review
the "local files versus databases"
section at the top of this document!

Line 12: The commented out entry (text after #) shows how
one would inform Postfix of the existence of a catch-all address.
Again, the lookup result is ignored by Postfix.

NEVER put a virtual MAILBOX wild-card in the virtual ALIAS
file!!

Note: if you specify a wildcard in virtual_mailbox_maps, then
you still need to configure the non-Postfix mailbox store to receive
mail for any address in that domain.

Lines 5, 15, 16: As you see above, it is possible to mix
virtual aliases with virtual mailboxes. We use this feature to
redirect mail for example.com's postmaster address to the local
postmaster. You can use the same mechanism to redirect any addresses
to a local or remote address.

Line 16: This example assumes that in main.cf, $myorigin
is listed under the mydestination parameter setting. If that is
not the case, specify an explicit domain name on the right-hand
side of the virtual alias table entries or else mail will go to
the wrong domain.

Execute the command "postmap /etc/postfix/virtual" after
changing the virtual file, execute "postmap /etc/postfix/vmailbox"
after changing the vmailbox file, and execute the command "postfix
reload" after changing the main.cf file.

Some providers host domains that have no (or only a few) local
mailboxes. The main purpose of these domains is to forward mail
elsewhere. The following example shows how to set up example.com
as a mail forwarding domain:

Line 2: The virtual_alias_domains setting tells Postfix
that example.com is a so-called virtual alias domain. If you omit
this setting then Postfix will reject mail (relay access denied)
or will not be able to deliver it (mail for example.com loops back
to myself).

Lines 3-11: The /etc/postfix/virtual file contains the
virtual aliases. With the example above, mail for postmaster@example.com
goes to the local postmaster, while mail for joe@example.com goes
to the remote address joe@somewhere, and mail for jane@example.com
goes to the remote address jane@somewhere-else. Mail for all other
addresses in example.com is rejected with the error message "User
unknown".

Line 10: The commented out entry (text after #) shows how
one would implement a catch-all virtual alias that receives mail
for every example.com address not listed in the virtual alias file.
This is not without risk. Spammers nowadays try to send mail from
(or mail to) every possible name that they can think of. A catch-all
mailbox is likely to receive many spam messages, and many bounces
for spam messages that were sent in the name of anything@example.com.

Execute the command "postmap /etc/postfix/virtual" after
changing the virtual file, and execute the command "postfix
reload" after changing the main.cf file.

More details about the virtual alias file are given in the
virtual(5) manual page, including multiple addresses on the right-hand
side.

The examples that were given above already show how to direct
mail for virtual postmaster addresses to a local postmaster. You
can use the same method to direct mail for any address to a local
or remote address.

There is one major limitation: virtual aliases and virtual
mailboxes can't directly deliver to mailing list managers such as
majordomo. The solution is to set up virtual aliases that direct
virtual addresses to the local delivery agent:

This example assumes that in main.cf, $myorigin is listed under
the mydestination parameter setting. If that is not the case,
specify an explicit domain name on the right-hand side of the
virtual alias table entries or else mail will go to the wrong
domain.

More information about the Postfix local delivery agent can be
found in the local(8) manual page.

Why does this example use a clumsy virtual alias instead of a
more elegant transport mapping? The reason is that mail for the
virtual mailing list would be rejected with "User unknown". In
order to make the transport mapping work one would still need a
bunch of virtual alias or virtual mailbox table entries.

In case of a virtual alias domain, there would need to be one
identity mapping from each mailing list address to itself.

This delivers mail to the recipient, and sends a copy of the
mail to the address that produces automatic replies. The address
can be serviced on a different machine, or it can be serviced
locally by setting up a transport map entry that pipes all mail
for autoreply.mydomain.tld into some script that sends an automatic
reply back to the sender.